Abstract: Apparatus is disclosed for testing an electrical circuit by means of signature analysis. Responses to a sequence of test patterns from the circuit under test are supplied to a linear feedback signature register (LFSR) which produces a signature signal at its output representing its current state in dependence upon its prior state and the received response signal. A programmed read-only-memory is addressed by these state signals of the LFSR and produces, at its output, a logical "1" signal if the current signature represents a permissible state of the LFSR and a logical "0" if the state is not permissable. This checking occurs throughout and during the testing sequence in contrast to conventional signature analysis wherein the comparison only occurs at the end of the testing sequence.